1. Field of the Invention
This invention relates to a sealing cover for use on an hermetically sealed container, especially one including a semiconductor device.
2. Discussion of the Prior Art
As is well known, it has become conventional to hermetically seal a semiconductor device in the cavity of a metallic or ceramic body to protect the device from adverse atmospheric effects and to provide physical protection. In the case of a ceramic body, a metallic ring is usually embedded in or fused into the body surrounding the cavity containing the semiconductor device. A metalic lid, to which has been attached a preformed ring of heat fusible material (solder), is placed over the cavity such that the fusible ring overlays the metallic ring on the ceramic body. The assembly is then clamped together and heated to form an hermetically sealed container for the semiconductor device.
In the embodiment of this design most popular in the semiconductor industry, the lid is made of Kovar (registered trade mark for a cobalt-nickel-iron alloy) or Alloy 42, (an iron--42% nickel alloy) which has been plated first with nickel and then with gold, and the solder is a gold-tin eutectic, while the outermost layer of the metallic ring on the ceramic body is also of gold.
Although Kovar is a suitable material for such lids by virtue of its coefficient of expansion being close to that of most ceramics, it has rather poor environmental resistance. For this reason, the gold plating is necessary both to prevent corrosion and also to provide a surface on the lid which is wettable by the gold-tin eutectic solder. However, recent decreases in the costs of manufacture of the semiconductor devices themselves and increases in the price of gold are making the use of precious metal plating and solders progressively less economic.
On the other hand, it is generally considered essential that the semiconductor device be hermetically sealed into its container and, consequently, that the materials of the container be environmentally resistant. It is unfortunately true that the mechanism protecting most metals and alloys (other than precious metals) from environmental and chemical attack also renders the surfaces of these metals resistant to effective cleaning by fluxes and thus relatively difficult to solder. Alternative approaches have been proposed, including the use of ceramic lids (CERDIP), and the use of shape memory alloys such as Tinel (a nickel-titanium alloy) (see, e.g., U.S. Pat. No. 4,126,758, assigned to the assignee of the present application). However, none of these alternatives have achieved commercial success and, despite the increase in cost, gold plated lids are still the norm in the semiconductor industry.
Further, it is still conventional in the art that the solder ring (mentioned previously) be manufactured separately from the lid and attached thereto, for example by spot welding. This approach suffers from the disadvantages that the solder ring may relatively easily separate from the lid proper, thus resulting in waste, and that substantial processing (melting, rolling, stamping, etc.) is required to form the ring.